Use a dropdown / select field instead of Radio button on a Donation form?

It should, there have been similar implementations in that fashion (i.e. custom button instead of dropdown), as example here's ours:

https://donations.diabetes.org/site/Donation2?df_id=19806&mfc_pref=T&19806.donation=form1


You could hide the default radio button through CSS, and create your dropdown and you need Javascript (i.e. jQuery) to perform the DOM on that dropdown html element to act as event trigger (on change) listener that would tell the system which radio button to be 'clicked' upon user selection on the dropdown. 


regards,

Daniel